The present invention relates to a variable speed mower drive mechanism comprising a friction disc transmission having a pair of discs with a friction or driven disc engaging the face of a drive disc. Friction disc transmissions of the type disclosed herein are used primarily for propulsion of small land traversing machines, particularly power lawn mowers.
A variable speed friction disc transmission of the type disclosed herein is shown in U.S. Pat. Nos. 4,169,392 and 4,173,153, assigned to the assignee of the present invention. The transmission disclosed in the aforementioned patents includes a driving disc rotatable about a vertical axis having a flat horizontal face normal to its axis of rotation and a driven disc rotatable on an axis normal to the axis of the driving disc and having its periphery engaging the face of the driving disc. The driving disc is rotatably supported on a swing arm which is swingable about a vertical axis.
The speed at which rotation is imparted to the driven disc is a function of the distance between the axis of the driving disc and the zone at which its face is engaged by the periphery of the driven disc. Further, the driven disc rotates in one direction on one side of the rotational axis of the driving disc and in the opposite direction on the opposed side.
The friction disc transmission disclosed in U.S. Pat. Nos. 4,169,392 and 4,173,153 has a self-energizing feature that causes the contact pressure between the driven and driving discs to be proportional to the torsional load on the wheel axle. However, the initial engagement between the driven and driving discs produces slippage which is a source of wear on the friction discs. Thus, there has been a need for a mower drive mechanism which provides a smooth start when the friction discs are initially engaged while being self-energizing to overcome slippage under load. Further, there has been a need for a friction disc transmission which minimizes the lateral resisting forces between the discs to allow the transmission to change ground speed or direction with ease and to provide, when required, additional traction force to the transmission to prevent slippage and to eliminate traction surging in all operating modes.